For a very long time there have been efforts to produce environmentally stable coatings and devices having very low reflectivity for a variety of industrial and scientific applications. These are important in imaging systems, calibration targets, instrumentation, light guides, baffles, stray light suppression and in many other uses.
To be commercially useful, these coatings must have the lowest reflectance possible and be capable of substantially uniform optical absorption over a wide area. Equally importantly, they should preferably exhibit a flat spectral response, low outgassing when exposed to vacuum, high resistance to mechanical shock and vibration with low particulate fallout, good thermal shock resistance and resistance to moisture. These are key requirements for many industrial and scientific applications as the coatings are often local to high sensitivity electronic detectors such as CCD (charged coupled device) or microbolometers. Any contamination from such coatings will inevitably collect or condense on the detectors rendering them faulty or lowering their performance beyond an acceptable threshold.
Until recently, the best black absorber coatings have achieved a total hemispherical reflectance (THR) of around 1.5%, although some experimental studies have achieved better using aligned carbon nanotubes (CNTs). For example, one group managed to achieve a THR of 0.045%. Generally, most aligned carbon nanostructure absorbers have THR's of around 0.5% to 1% in the mid-infra-red (IR) region of the electromagnetic spectrum. When grown at a commercial scale it has proven difficult to reproduce the best performance from aligned carbon nanostructure coatings with any consistency on commercial substrates.
Aligned carbon nanotubes and filaments have been shown to be highly efficient absorbers of electromagnetic energy, and also satisfy many of the key requirements for super-black optical absorbers listed above, but they are prone to attack by atmospheric moisture and humidity. This is caused by growth defects in the carbon nanostructures, which become more pronounced as their synthesis temperature drops below 900° C. As most super-black coatings made from carbon nanotubes are required to be used on sensitive instrumentation, they tend to be grown at the lowest possible temperature that is compatible with the substrate. The chemical vapour deposition (CVD) methods used to grow these coatings tend to result in them being very hydrophilic, as growth defects in the tube walls terminate to form highly polar hydroxyl, carbonyl and carboxyl functional groups on exposure to air or trace oxygen. This hydrophilicity rapidly causes the film to lose its optical properties on exposure to atmospheric humidity or free water, as the film loses its intended highly-absorbing structure and acts like a sponge.
A method that helps to improve the electromagnetic absorption performance of aligned nanostructure films is that of post growth oxygen plasma etching. This has the effect of clustering the nanotube tips, creating a more open structure and a rougher surface topography. This type of post growth process can improve the absorber's performance by over 30% in many cases. However, coatings made from carbon nanostructures that have been treated with an oxygen or other inert gas plasma are highly defective due to ion damage of the nanostructure shells. This leads to the nanostructure coating becoming more hydrophilic so, again, when the absorber coating is exposed to atmospheric humidity or water, severe, full length, CNT tube agglomeration rapidly occurs as the moisture is attracted to the highly polar tube defect sites. This causes uniform local areas of tube densification distributed across the surface exposed to atmospheric moisture. This in-turn radically decreases the coating's ability to act as an absorber, making the film unsuitable for practical applications.
Approaches by research groups who wish to make self-cleaning, super-hydrophobic surfaces have focused on the use of polymer coatings on carbon nanotubes and filaments. Whilst conventional hydrophobic coatings would protect the nanotube coating from water and humidity, they also drastically reduce the combined polymer/absorber film's absorbing properties by changing the film's refractive index, blocking the existing optical trapping cavities and reducing the photon absorption efficiency of the individual nanotubes or filaments.
Metals, semiconductors and materials with metallic behaviour, such as CNTs, demonstrate bandgaps which are generally small enough for an electron to be promoted to an excited state by the energy of an incoming photon. This excitation, followed by extremely fast relaxation through π/π* interactions inherent to the structure of graphitic carbon, enables the large uptake of energy observed in these materials. The energies at which these transitions can occur is dependent on CNT structure, size, and environment (i.e. tangled or isolated) and as such can be tuned to occur over a wide spectrum given the correct combination.
Most polymers are insulators, which means that their band gap is too large for an electron to be excited easily by a photon. This means that photons will simply be transmitted, reflected or diffused by the polymer depending on its refractive index and surface roughness.
Molecules and atoms are excited only when the energy of an impinging photon matches the energy difference between the state in which the molecule initially finds itself and some higher energy state of the molecule. To change from a lower quantized energy level to a higher one, the energy of the photon must match the energy gap between the levels. In equation form we can writeElower state+Ephoton=Eupper state That is, in order for light absorption to take place:Ephoton=hν=ΔEmolecule=Eupper state−Elower state 
Carbon nanostructures demonstrate a feature of one dimensional materials termed Van Hove singularities. These are regions with a high density of states where multiple electronic transitions are possible, causing a very sharp and strong absorption in a narrow band of energy. Metallic and semiconducting CNTs (as well as other 1D materials) demonstrate band gaps (Van Hove singularities) highly dependent on their structure, size and environment. The nature of a vertically aligned forest such as that described in the references below, results in a large distribution of CNT size, length, alignment and crystal structure and hence wide band absorption. It is also known that interactions between nanotubes, such as bundling, results in a wider frequency absorption range. The nature of the bonding in CNTs involving electron delocalisation allows for fast transitions between excited and relaxed states and allows for absorbed energy to be easily dissipated through heat which results from vibrations. When a CNT is over coated in polymer material it makes this absorption far less efficient, resulting in far higher reflectance from the coating.
To date, these issues have not been addressed or resolved in aligned, carbon nanotube or filament electromagnetic (EM) absorber coatings.
One example of an aligned absorber is: US patent application: 20090126783 A1 by Shawn-Yu Lin et al of Rensselaer Polytechnic Institute, entitled: Use of vertical aligned carbon nanotube as a super dark absorber for pv, tpv, radar and infrared absorber application. This publication discloses a visible spectrum highly absorbing aligned carbon nanotube film. The aligned array absorbers are grown using conventional chemical vapour deposition (CVD) techniques at high temperatures >750° C.
A study by N A Tomlin et al, “Decrease in Reflectance of Vertically-Aligned Carbon Nanotubes after Oxygen Plasma Treatment”, Carbon Journal (Elsevier) vol. 74, pp. 329-332, August 2014, has suggested that a low reflectivity coating formed of vertically aligned carbon nanotubes could exhibit decreased reflectivity after oxygen plasma treatment.
A study by Kenneth K. S. Lau et al in the Journal Nano Letters, entitled: “Super Hydrophobic Carbon Nanotube Forests”, discusses the effect of coating carbon nanotubes in a fluoropolymer to make the surface hydrophobic.
Prior art documents which are not relevant to patentability of the present invention include US2014342954A; FR2887872A; US2008170982A; US2013230695A; US2014011013A; US2014342098A; US2014342103A; US2015173883A; US2012121916A; US2012241687A; US2012276335A; US2013089807A; US2007172666A; US2008118734A; US2009104347A; WO2013/009684 A1 (U.AKRON); CN104631093 A (YANCHENG); US2007/0110982 A1 (PLISSONNIER); Tomlin, Curtin, White, Lehman, “Decrease in reflectance of vertically-aligned carbon nanotubes after oxygen plasma treatment”, Carbon, 2014, volume 74, pages 329-332, Elsevier; Lau, Bico, Teo, Chhowalla, Amaratunga, Milne, McKinley, Gleason, “Superhydrophobic carbon nanotube forests”, nano letters, 2003, volume 3, number 12, pages 1701-1705, American chemical society; US2009/0050601 A1 (PARK); WO2009/083562 A1 (ESSILOR); US2010/0285301 A1 (DIEUDONNE); and Xie, Wang, Cui, Shi, “No—Fe—Co—P coatings on coiled carbon nanofibers”, Letters to the Editor, Carbon, 2005, volume 43, pages 3181-3183, Elsevier.